Currency unions are a tricky business; a successful one requires integrated markets, labour mobility, mechanisms for fiscal transfers and centralised financial supervision. The US shows the effectiveness of a union that works.

The Eurozone does not meet these criteria and the situation in Greece illustrates the dangers of a sub-optimal union. While a temporary agreement has been reached, Grexit risk has been delayed but not eliminated.

Staying out of the Eurozone appears to have been a lucky escape for the UK. Looser ECB policy before the crisis would have stoked deeper imbalances, while the policy response since has been insufficient.

Movements in the yen over recent years look to be closely related to policy decisions in Tokyo and Washington. The Bank of Japan's Kuroda has downplayed the role that QE has played as domestic unease with a weak yen intensifies.

Weak economic fundamentals in China are causing concern that currency devaluation is in the works. However, the high import content of China's exports and capital outflow risks make devaluation less attractive.